Photocatalytic activity of the biogenic mediated green synthesized CuO nanoparticles confined into MgAl LDH matrix

The global concern over water pollution caused by organic pollutants such as methylene blue (MB) and other dyes has reached a critical level. Herein, the Allium cepa L. peel extract was utilized to fabricate copper oxide (CuO) nanoparticles. The CuO was combined with MgAl-layered double hydroxides (MgAl-LDHs) via a co-precipitation method with varying weight ratios of the CuO/LDHs. The composite catalysts were characterized and tested for the degradation of MB dye. The CuO/MgAl-LDH (1:2) showed the highest photocatalytic performance and achieved 99.20% MB degradation. However, only 90.03, 85.30, 71.87, and 35.53% MB dye was degraded with CuO/MgAl-LDHs (1:1), CuO/MgAl-LDHs (2:1), CuO, and MgAl-LDHs catalysts, respectively. Furthermore, a pseudo-first-order rate constant of the CuO/MgAl-LDHs (1:2) was 0.03141 min−1 while the rate constants for CuO and MgAl-LDHs were 0.0156 and 0.0052 min−1, respectively. The results demonstrated that the composite catalysts exhibited an improved catalytic performance than the pristine CuO and MgAl-LDHs. The higher photocatalytic performances of composite catalysts may be due to the uniform distribution of CuO nanoparticles into the LDH matrix, the higher surface area, and the lower electron and hole recombination rates. Therefore, the CuO/MgAl-LDHs composite catalyst can be one of the candidates used in environmental remediation.


Preparation of plant extract
The Allium cepa L. peel plant extraction process was performed according to literature report with modification 40 .In a particular procedure, the Allium cepa L. peels were washed with distilled water several times before being dried at 85 °C.The dried Allium cepa L. peels were then cut into small pieces.Then, five grams of the peel was transferred to a flask and 350 mL of distilled water was added.The mixture was stirred and boiled at 70 °C for 30 min to get an aqueous extract.Then, the solution was cooled at room temperature and filtered through filter paper.

Synthesis of CuO nanoparticles (CuO NPs)
In the preparation of CuO NPs, copper nitrate trihydrate (Cu(NO 3 )•3H 2 O), sodium hydroxide (NaOH), and Allium cepa L. peel extract were used based on the literature report with minor modifications 22 .Specifically, 1.208 g of Cu (NO 3 ) 2 •3H 2 O was dissolved into 100 mL of distilled water.Then, the resulting solution was dropped into an aqueous extract of Allium cepa L. peel (100 mL) under stirring for 45 min at room temperature.Then, aqueous solution of NaOH (0.5 mol L −1 ) was added to attain a pH of 10.The entire reaction was carried out at 70 °C for 3 h with constant stirring.The solution was aged for 24 h before being centrifuged and dried overnight at 80 °C.Finally, the powder sample was calcined at 400 °C for 2 h.

Preparation of MgAl-LDHs
The MgAl-LDHs preparations were carried out using a co-precipitation method with Mg 2+ /Al 3+ molar ratio of 3:1 according to a literature report with modification 41 .Briefly, an aqueous solution containing NaOH (4 g)

Preparation of CuO/MgAl LDHs nanocomposites
Co-precipitation was used to prepare the CuO/MgAl-LDHs (1:2) composite catalyst.To obtain a uniform suspension, the prepared CuO (0.3077 g) was ultrasonically dispersed in 100 mL distilled water for 1 h and then placed in a 60 °C water bath with stirring.Then, a salt solution containing 0.192 g MgCl 2 •6H 2 O and 0.2424 g Al(NO 3 ) 3 •9H 2 O were added to the suspension.To keep the pH at 10.5, an aqueous solution containing NaOH (4 g) and NaHCO 3 (6.30g) was added drop-wise.The precipitate was aged at 60 °C for 12 h and then washed with deionized water until the solution became neutral.Finally, the composite was dried for 12 h at 80 °C42 .For comparison purposes, different weight ratios of the CuO/LDHs abbreviated as CuO/MgAl-LDHs (1:2), CuO/ MgAl-LDHs (1:1), and CuO/MgAl-LDHs (2:1) were prepared.The preparation of the composite catalyst is shown in Scheme 1.

Photocatalytic activity
The photocatalytic activities of the catalysts were checked by the degradation of MB dye.In the degradation process, 25 mg of the catalyst was added into 100 mL (10 mg L −1 ) of MB dye solution and stirred for 30 min to establish an adsorption-desorption equilibrium in the dark.Then, the suspension was exposed to a 150 W halogen lamp for 80 min under regular stirring.To analyze the degradation of the dye, 5 mL of the aliquot was taken every 20 min of reaction time followed by centrifugation.The absorbance at a maximum wavelength of 664 nm was used to determine the concentration of the dye leftover.The radicals scavengers effects in the degradation process of MB dye were performed and EDTA-2Na, isopropyl alcohol, and AgNO 3 were used for trapping of h + , •OH, and e − , respectively 43,44 .The stability of the CuO/MgAl-LDHs (1:2) catalyst was also checked according to the literature report with modification 45 .After each cycle, the catalyst was recovered by centrifugation, washed with deionized water, and dried in an oven at 80 °C.

Characterizations
X-ray diffractometer (Shimadzu XRD-7000) with Cu Kα radiation (λ = 1.5406Å) operating in the range of 2θ = 10° to 80, 30.0 mA applied current, and 40.0 kV acceleration voltage was used to check the crystallinity and phases of the samples.The average crystallite size (D) and interplanar spacing (d) were calculated according to Debye-Scherrer equation and Bragg's Law, respectively 46,47 .Fourier transforms infrared spectroscopy (FTIR-6600 type A) was used to check bonding and functional groups of the samples in the range 4000-400 cm −1 .The morphologies of the samples were examined by field-emission scanning electron microscopy (FE-SEM, JSM 6500F, JEOL) and transmission electron microscopy (TEM) (FEG TEM Tecnai G2 F30).X-ray photoelectron spectroscopy (XPS) (ESCALAB 250) was used to examine the chemical states of the sample.The JASCD FB-8500 photoluminescence (PL) spectroscopy was used to check the electron and hole separation rates.A Shimadzu-3600 plus UV-vis spectrophotometer was used to analyze the concentration of MB dye at a maximum wavelength of 664 nm.JASCD V-670 UV-visible-near-infrared (UV-vis-NIR) spectrophotometer was used to measure UV-vis diffuse reflectance spectra (DRS) using BaSO 4 as a reference.113), (311), and (004), respectively, for CuO (Fig. 1a).The existence of all peaks correspond to the monoclinic CuO phase which is consistent with the reported research (JCPDS No. 00-048-1548) 48 .Moreover, the average crystallite size for CuO was also calculated and showed 16.1 nm with an interplanar spacing of 0. The XPS analysis was used to investigate the chemical states of the elements in the CuO/MgAl-LDHs (1:2) composite sample.The peaks located at 933.8 and 953.8 eV, correspond to Cu 2p 3/2 and Cu 2p 1/2 , respectively, which indicates the presence of Cu 2+ in the CuO (Fig. 2a).Furthermore, the broad satellite peaks at a higher binding energy provided additional confirmation of the CuO.The Cu 2p was accompanied by two satellite peaks on the higher binding energy side at 943.8 eV and 941.5 eV, indicating the presence of CuO 51 .Moreover, the satellite peak at around 943.8 eV clearly demonstrates an open 3d 9 shell, thus, supporting the presence of Cu 2+ in the sample, which is positioned at higher binding energies than the main peaks 52,53 .Moreover, the highresolution spectrum of the Mg 2p is also presented in Fig. 2b.In the Mg 2p spectra, a single peak appears at 49.6 eV, corresponding to Mg 2+ in the brucite layers of MgAl-LDHs 54 .The Al 2p spectrum, deconvoluted into two peaks at 75.15 and 77.15 eV, can be assigned to Al 2p (α) and Al 2p (β) of Al(OH) 3 and Al 2 O 3 , respectively, emerging from the LDHs structure, as shown in Fig. 2c 55 .Similarly, the binding energy at 529.4 eV belongs to lattice oxygen in the O 1s spectrum, while the peak at 531.6 eV reflects chemisorbed oxygen and coordinated lattice oxygen (Fig. 2d) 56 .
The FE-SEM was used to evaluate the morphological properties of the CuO NPs, MgAl-LDHs, and CuO/ MgAl-LDHs (1:2) samples as shown in Fig. 3a-c.As it was observed in Fig. 3a, the CuO NPs had a spherical-like morphology.The biogenic-mediated preparation makes the CuO nanoparticles smaller and spherical morphology with different sizes.The smaller particles agglomerate and organize themselves into larger spheres 57 .In the preparation, the presence of functional groups in the plant extract encourages dynamic behavior during nucleation and stabilization 58 .Figure 3b  analyzed through the EDS analysis (Fig. 3d).The EDS spectrum of CuO/MgAl-LDHs (1:2) revealed the presence of Mg, Al, O, and Cu elements.
To understand the morphology and microstructure of the sample in detail, the TEM analysis was performed.The TEM and HRTEM of the CuO/MgAl-LDHs (1:2) nanocomposite are indicated in Fig. 4. TEM image confirms that dark spots of CuO nanoparticles are distributed in the matrix of LDHs (Fig. 4a).Moreover, the d-spacing of the particles adhered to the surface of the LDH support is 0.234 nm, as shown from the HRTEM image (Fig. 4b), which belongs to the CuO (111) plane.The results revealed that the composites of the nanosized CuO-LDHs matrix were formed.Zeta potential (ZP) measurements were also performed to check the stability of the particles suspensions and their surface charge properties 59 .The average zeta-potential values for CuO, MgAl-LDHs, and CuO/MgAl-LDHs (1:2) catalysts were + 15.45 mV, + 32.7 mV, and + 19.75 mV, respectively, and shown in Fig. 1S (Supplementary).The lower dispersions ZP values will a tendency of coagulation, aggregation, or flocculation due to van der Waals interparticle attraction 60 .After CuO was incorporated into LDHs, the dispersion of ZP f was changed to + 19.75 mV, indicating that the CuO particles were well dispersed into MgAl-LDHs matrix which is also confirmed TEM analysis (Fig. 4).
The functional groups of the as-prepared CuO NPs, MgAl-LDHs, and CuO/MgAl LDHs (1:2) samples were investigated by FTIR spectroscopy.As shown in Fig. 5a,b, the O-H stretching vibrations were represented by an absorption band at 3427 cm −1 .An asymmetric stretching of C-O occurs at 1244 cm −1 resulting in cyclic polyphenol compounds 61 .In Fig. 5c, the stretching modes of the O-H groups associated with metal cation bonds in the hydroxide layer and the stretching vibrations of interlayer water molecules, attributed to the strong and broad absorption peak at 3440 cm −1 for pure MgAl-LDHs 42 .The asymmetric vibration of CO 3 2− anions and the bending vibration of water molecules between layers in the interlayer region affected the peaks at 1345 cm −1 and 1622 cm −1 , respectively, while the presence of C-O and C=O bonds might cause the CO 3 2− peak to break 35,62   sample could be due to the visible light-responsive property of CuO.Although CuO had higher absorbance in the visible light region, the photocatalytic performance could be lower due to the higher electron and hole recombination rates.However, significant adsorption intensity enhancement of the MgAl-LDHs may be attributed to the quantum effect of CuO which will inhibit the electron-hole pair's recombination rate that facilitates the degradation efficiency.
The PL emission spectra of CuO NPs, MgAl-LDHs, and CuO/MgAl-LDHs (1:2) composite were performed at 300 nm exited energy and are shown in Fig. 6b.The PL emission peak for the CuO/MgAl-LDHs (1:2) is substantially reduced, indicating a higher separation efficiency of photoinduced charge carriers than bare samples 54 .The rate of the recombination of excited electron-hole pairs determines the intensity of PL emission.The higher intensity indicates a faster recombination rate, while the lower intensity indicates a large amount of transferred

Photocatalytic activities
The photocatalytic performances of the CuO NPs, MgAl-LDHs, CuO/MgAl-LDHs (1:1), CuO/MgAl-LDHs (1:2), and CuO/MgAl-LDHs (2:1) composite samples were tested in the degradation of MB dye.The UV-vis absorption spectra for the MB degradations are shown in Fig. 7.As demonstrated in Fig. 7a-e, the time-dependent dye absorption intensity was decreased after the photocatalytic reaction was performed.The higher photocatalytic performance was observed in the presence of CuO/MgAl-LDHs (1:2) sample than other samples within 80 min irradiation time.The results also indicated that the addition of the optimum amounts of CuO in the composite preparation system can affect the photocatalytic properties.It can be also demonstrated that the composite catalyst showed better performances than bare CuO NPs and MgAl-LDHs samples.It could be due to interfacial charge transfers which facilitate the electron migration and suppression of electron and hole recombination 41 .
Moreover, the degradation performance was also demonstrated as shown in Fig. 8a.The degradation of MB was almost negligible throughout the catalyst (blank).The photodegradation efficiency of MB with CuO/ MgAl-LDHs (1:2) was 99.20%.However, the MgAl-LDHs, CuO NPs, CuO/MgAl-LDHs (2:1), and CuO/MgAl-LDHs (1:1) catalyst degradation efficiencies were 35.53, 71.87, 85.30, and 90.03%, respectively.Furthermore, the MgAl-LDHs demonstrated high adsorption of MB molecules in the dark.It can be attributed to MgAl LDHs  Figure 8b,c provides detailed information on the kinetic parameters of pseudo-first-order fitting for MB degradation 66 .The degradation rate constant (k) for CuO/MgAl-LDHs (1:2) was estimated to be 0.0314 min −1 , while the kinetic rates for CuO, LDHs, CuO/MgAl-LDHs (2:1), and CuO/MgAl-LDHs (1:1) samples were 0.0156, 0.0052, 0.0234, and 0.02855 min −1 , respectively.The CuO/MgAl-LDHs (1:2) kinetic value was higher than that of all other samples.The findings showed that the CuO/MgAl-LDHs (1:2) composite had the best photocatalytic activity in visible light irradiation.Moreover, the catalytic performance of the composite catalyst was also compared with other literature reports.As it is demonstrated in Table S1 (Supplementary), the CuO/MgAl-LDHs (1:2) composite catalyst was comparable with other catalysts in the degradation of organic pollutants.Figure 8d also demonstrates the stability of the CuO/MgAl-LDHs (1:2) composite catalyst.The CuO/MgAl-LDHs (1:2) composite catalyst showed higher photocatalytic stability after five cycles.The slightly reduced activity was primarily due to the mass loss of the photocatalyst during centrifugation to collect the reused catalyst.After five cycles, 92.7% of the MB dye degradation was maintained.
The photocatalytic degradation mechanism was proposed according to the experimental findings and characterizations results.When the photon energy which is equal to or greater than the band gap energy strikes the surface of the CuO/MgAl-LDHs, the electrons will be ejected from the balance band and moved to the conduction band 67 .The CuO creates electron-hole pairs and the positive charges on the surface of LDHs platelets can attract electrons (e − ) produced by the CuO particles, while the holes (h + ) move to the opposite direction, leading to the separation of the photogenerated e − and h + pairs 34 .The photogenerated holes (h + ) reacted with hydroxyl ions to produce hydroxyl radicals (•OH), and the •OH radicals will oxidize the organic contaminants 68 .Furthermore, the LDH layer's surface − OH groups reacted with valence band holes to produce hydroxyl radicals (•OH), which are significant variables in photo-oxidation reactions 39 .Similarly, the holes can directly interact with organic pollutants and degradation will be facilitated.According to the experimental finding of scavenging effects (Fig. S2) (Supplementary), the degradation efficiency in the presence of isopropanol (•OH scavenger) significantly decreased.However, in the presence of EDTA-2Na (h + scavenger) and AgNO 3 (e − scavenger), the degradation efficiencies were better than in the presence isopropanol.As it was demonstrated from trapping experiments, the catalyst performance without scavengers was much higher than that of the degradation in the presence of scavengers.Simultaneously, superoxide radicals ( • O 2 − ) were formed when excited electrons were interacted by dissolved oxygen species in an aqueous solution 67 .Organic matter could be decomposed into CO 2 and H 2 O after interaction with reactive h + , • O 2 − and •OH species 69 .Therefore, the •OH, • O 2 − , h + , e − reactive species can be responsible for the degradation of MB dye using CuO/MgAl-LDHs nanocomposites.Moreover, the CuO NPs, which are effectively distributed over the LDH and their synergistic impact in quick dye adsorption followed by rapid photodegradation are attributed to the photocatalytic improvement of the CuO/MgAl-LDHs nanocomposites.It should be highlighted that the strong interaction with LDH aided charge transport and improved the photocatalytic function of composites.The possible degradation mechanism is depicted in Fig. 9.

Conclusions
The CuO semiconductor catalyst synthesized with a green method was combined with MgAl-LDH and applied for the degradation of MB dye.The catalytic performance of the CuO/MgAl-LDHs (1:2) composite exhibited the highest catalytic performance and degraded 99.20% MB dye within 80 min.However, only 90.03, 85.30, 71.87, and 35.53% of MB dye degradation was achieved by CuO/MgAl-LDHs (1:1), CuO/MgAl-LDHs (2:1), CuO NPs, and MgAl-LDHs catalysts, respectively.The enhanced photocatalytic activity was attributed to a synergistic effect, homogeneous dispersion of CuO NPs, adsorption of MB on MgAl-hydroxyl-rich LDH's structure, and lower band gap of CuO NPs.The catalyst stability was also checked and 92.7% of the MB dye removal efficiency was still maintained after five cycles.Therefore, the CuO/MgAl-LDHs-based photocatalyst could be a potential candidate for the environmental remediation.
. The vibrations of M-O/M-O-M (M = Mg, Al) characteristic of the double-lamellar structure are responsible for the remaining bands below 63 1000 cm −1 .The CuO/MgAl-LDHs (1:2) spectrums comprised all feature bands of CuO NPs and MgAl-LDHs, as shown in Fig. 5d.The optical properties of the CuO NPs, MgAl-LDHs, and CuO/MgAl-LDHs (1:2) samples were examined and shown in Fig. 6a.The absorbance of the MgAl-LDHs was lower than that of CuO and CuO/MgAl-LDHs (1:2).However, the absorbances of the CuO NPs and CuO/MgAl-LDHs samples were higher as compared to bare MgAl-LDHs.The results indicated that the enhanced absorption of the CuO/MgAl-LDHs composite

Figure 8 .
Figure 8.(a) The C/C 0 versus irradiation time plot of the MB degradation with different catalysts, (b,c) the kinetics study of different catalysts, and (d) the stability of CuO/MgAl-LDHs (1:2) sample.